1. Field of the Invention
The present invention relates to an improved process for selective, energy-saving thermal after-treatment of cleavage product from the acid-catalyzed cleavage of cumene hydroperoxide (CHP) into phenol and acetone.
2. Description of the Background
The process of acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone has long been of particular industrial importance. In the preparation of phenol from cumene by the Hock process, cumene is oxidized to cumene hydroperoxide (CHP) in a first reaction step, known as oxidation, and the CHP is subsequently concentrated to 65 to 90% by weight in a vacuum distillation, known as concentration. In a second reaction step, known as cleavage, the CHP is cleaved into phenol and acetone in the presence of an acid, usually sulfuric acid. Here, the dimethyl phenyl carbinol (DMPC) formed in the oxidation is partly dissociated into xcex1-methylstyrene (AMS) and water in an equilibrium reaction, and a further part of the DMPC reacts with CHP to form dicumyl peroxide (DCP), while the remainder of the DMPC remains in the cleavage product. After neutralization of the cleavage product, this product mixture is worked-up by distillation.
During the cleavage, part of the AMS forms high boiling compounds, i.e. high boilers such as dimers and cumylphenols, which are discharged as a residue from the distillation apparatus. The AMS still present after the neutralization is hydrogenated to cumene during the distillation and is recirculated to the oxidation step. DMPC which has not reacted in the cleavage reaction passes as a high boiler to the residue, and part of it reacts further in the hot phenol columns to form AMS from which high-boiling secondary components are in turn formed. The DCP is stable at customary cleavage temperatures (50 to 70xc2x0 C.). In the hot phenol columns, it decomposes thermally to form, in the inventor""s experience, primarily o-cresol. On the other hand, in the presence of acid, DCP can be cleaved into phenol, acetone and AMS at temperatures to above 80xc2x0 C. It is, therefore, obvious to react the residual DMPC and the DCP formed in the cleavage completely immediately after the cleavage by means of a targeted increase in the temperature in the presence of the acid used as catalyst in the cleavage.
In this way, DMPC is converted virtually completely into AMS, and DCP is converted completely into phenol, acetone and likewise AMS.
Such a thermal after-treatment of the cleavage product has already been described in U.S. Pat. No. 2,757,209, where temperatures above 100xc2x0 C., especially from 110 to 120xc2x0 C., were used. The objective of this thermal after-treatment is complete dehydration of the DMPC to AMS. In contrast, U.S. Pat. No. 4,358,618 describes a thermal after-treatment which has as its objective the complete conversion of the DCP formed in the cleavage into phenol, acetone and AMS, using temperatures ranging from 120 to 150xc2x0 C. U.S. Pat. No. 5,254,751 describes a thermal after-treatment with the same objective as U.S. Pat. No. 4,358,618, using temperatures ranging from 80 to 110xc2x0 C. Finally, in DE 197 55 026 A1, the after-treatment is conducted in a temperature range above 150xc2x0 C. Accordingly, the optimum temperature ranges specified for the thermal after-treatment of cleavage product from phenol production differ widely in the disclosures hitherto.
In all these previously described processes, the cleavage product is first heated by means of steam in heat exchangers in order to conduct the thermal after-treatment and, after a sufficient reaction time, the product is cooled again by means of water in heat exchangers. Depending on the temperature selected for the thermal after-treatment, this gives specific steam consumptions of 0.2 metric tons of steam per metric ton of phenol. We have found that increased deposition of high-boiling by-products in the heat exchangers (fouling) of the thermal after-treatment generally occurs at temperatures above 100xc2x0 C., especially above 120xc2x0 C., and this fouling is associated with a drastic decrease in heat transfer. Particularly in the apparatuses for heating the product by means of steam, organic deposits form on the hot heat exchange surfaces on the product side, so that these apparatuses have to be cleaned at relatively short intervals of a few weeks. This fouling increases further as the temperatures increases. A need, therefore, exists for a process modification which reduces fouling.
Accordingly, one object of the invention is to provide a process for the thermal after-treatment of cleavage product from cumene hydroperoxide cleavage, which displays not only high selectivity but also low energy costs and a high availability because of the avoidance of fouling.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for the thermal after-treatment of cleavage product from the acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone which comprises heating the cleavage product in a reactor, wherein the heat supplied for the thermal treatment is the heat generated by at least one exothermic reaction which occurs in the reactor. The thermal treatment of the invention provides a high selectivity of the after-treatment combined with a lowering of the energy costs and a greater operating period of the heat exchangers because of the avoidance of fouling.